feat(web): cloud chat-model selector in Settings

Pick which OpenAI model answers on the Cloud backend (gpt-4o / -mini / 4.1 /
4.1-mini / o4-mini, or Default). Persisted in localStorage, sent as `model` in
the chat request; respond() applies it only on the cloud backend (local/mi50
keep their fixed models). Reachable from desktop + mobile via Settings.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyra/web/server.py

@@ -92,9 +92,10 @@ def create_app() -> FastAPI:
         backend = _backend_for(body.get("backend"))
         user_msg = _last_user_message(body.get("messages", []))
 
+        model_override = body.get("model") or None
         memory.ensure_session(session_id)
         try:
-            reply = await asyncio.to_thread(chat.respond, session_id, user_msg, backend)
+            reply = await asyncio.to_thread(chat.respond, session_id, user_msg, backend, model_override)
         except Exception as exc:
             logbus.log("error", "chat failed", session=session_id, error=str(exc))
             reply = f"[error] {exc}"